ECHINOIDEA. 



219 



absent, and in Pygaster the posterior 

 basal itself is absent. In the Clypea- 

 stroids there are usually five genital 

 openings, but sometimes that of the 

 posterior interradius is absent. In 

 Spatangoids there are never more than 

 four genital openings, that of the pos- 

 terior interradius being absent. In 

 many of the geologically older forms 

 the corresponding basal has also disap- 

 peared, but it is present in most recent 

 and living Spatangoids (p. 217), though 

 without a genital opening. In some 

 Spatangoids the genital opening of the 

 right anterior basal is also absent, and 



s o in e t i m e s 



nv 



IF 



V 



g 



FIG. 162. Apical system of 

 CoUyrites elliptica Lamk. 

 (from Lang, after Loven). 

 I-V the ambulacra num- 

 bered ; a, b the corre- 

 sponding rows of ambula- 

 cra lettered (see Loven's 

 law). 



FIG. 161. Apical system and adjacent 

 parts of Holaster suborbicularis Defr. 

 (after Loven, from Lang), m madre- 

 poritic basal ; r radials ; 6 basals ; 

 v radius v ; 2 and 5 2nd and 5th 

 interradius. 



that of the 

 left anterior 

 as well, so 

 that only two 

 genital open- 

 ings are left. 



In some Spatangoids the apical system pre- 

 sents still more curious variations. The typical 

 arrangement which is found in the Endocyclica 

 and in the Exocyclica described above, and in 

 which the plates are all grouped round a centre, 

 is called compact. But in certain Spatangoids 

 it may be elongated, i.e. the plates are arranged 

 in two rows extending in an antero-posterior 

 direction (Fig. 161). As a result of this the 

 rays of the trivium are removed by a consider- 

 able interval from those of the bivium. This 

 modification is carried still further in the Colly- 

 ritidae, in which the radials of the bivium are 

 separated, by the junction of the plates of the 

 right and left posterior interradii, from the 

 rest of the apical system (viz. 3 radials and 

 4 basals, the posterior basal being absent). 

 Such an apical system is said to be disjunct 

 (Fig. 162). 



Loven's law. In the preceding description of 

 the Echinoid shell, a certain enumeration of the 

 radii and orientation of the shell has been 

 adopted. One of the radii has been called ante- 

 rior, and one of the interradii posterior, while the 

 three anterior have been spoken of as the trivium 

 and the two posterior as the bivium. When the 

 madreporite is distinct, this orientation can be 

 easily determined ; the anterior radius being 

 that to the left of the madreporitic basal (Figs. 

 148, 161). The orientation is also easy when the 

 anus is outside the apical system, because it 



